1. Field of Invention
The present invention relates to improvements in apparatuses for the refining of pulp as well as to a method for the refining of pulp in a paper making process.
2. Description of Prior Art
Disc refining dates from antiquity with the preparation of food and drink. Modern disc patterns reflect that ancient art by still having patterns of many straight or slightly-curved lines that impact a paddle-like action on the material. The paddle-like action, at high speed, creates impacts accompanied by much noise, fast wear and energy loss.
Some of the known grinding mills utilize parallel cylinders with spiral teeth. Those on one cylinder intermesh with the teeth of the other, the teeth serving for one cylinder to drive the other. That is, the teeth act as helical gears, and material such as pulp is mashed at these gears. In other known apparatuses, the helical grooves or ribs of a rotor may have variable pitch for urging material along a smooth casing that is cylindrical or conical. Some helical apparatuses with uniform pitch specifically avoid intermeshing such as discussed in Krone's U.S. Pat. No. 3,197,147, Col 3, line 66. Such cylindrical and conical apparatuses have substantially co-axial rotors. Disc refining normally has co-axial positioning of the rotors, one disc either rotating opposite from the other or being held stationary or being rotated at a slower speed in the same direction. In each instance, refining is achieved by a difference of rotational speed between two discs, one disc rubbing the other through a layer of material being refined. Thus, two co-operating discs do not mesh together, and instead the difference of rotational speed creates shear planes that help to refine material. Known disc refining utilizes patterns of many short and substantially radial lines that impart a paddle-like action on material being refined.
The results of such refiner patterns are high noise levels, rapid wear and power loss. Heat generated from the resultant friction absorbs so much energy that a cooling system is commonly used. Substantially radial paths allow material to eject too readily centrifugally from the periphery of the discs.
Some disc apparatuses attempt to reduce this ejection by including small dams at various places between the radial ribs while other apparatuses are designed to reduce the effect of rapid centrifugal ejection by recirculation of the material being refined. However, recirculation requires enlargement of the apparatus to accommodate both main flow and recycled flow.